Represented in FIG. 1 is an exemplary simplified conventional architecture of an RF (radio frequency) transmitter, more particularly a Ka band transmitter (29-30 GHz). As represented in the figure, the RF signal lying in the 950-1450 MHz band corresponding to the frequency band at the output of the indoor unit (IDU) is amplified by a variable gain amplifier 10. The signal output by the amplifier 10 is transposed into Ka bands, namely the band lying between 29.5-30 GHz. This transposition is carried out by a subharmonique mixer 11 and a local oscillator 12 operating in the Ku band. The output of the mixer 11 is sent through an amplifier 12 as input to a filtering element 13 whose major constraint is the rejection of the spectral component at 2FOL where FOL is the frequency of the local oscillator. The value of 2FOL is, for example, equal to 28.55 GHz, namely to a value very close to the value of the transmission band in the Ka band (29.5-30 GHz). The signal arising from the filtering device 13 is sent to a second amplifier 14 and a power amplifier 15 before reaching the transmission antenna 16. This very simple structure allows direct transposition in a single leap. Its main advantage is that it is cheaper than double transposition architectures. Specifically, the number of components and the area occupied by the circuits are reduced. Nevertheless, this technique considerably increases the constraints on the filtering of the transmission band so as to satisfy the international standards in force.
To obtain the rejection for the 2FOL component, as mentioned hereinabove, various filtering devices may be used.
It is in particular possible to use waveguide filters, more particularly filters embodied in thin layer/alumina technology, which allow selective filtering, in particular in the K or Ka bands. However, this technology is expensive and is incompatible with SMC (surface mounted component) technology on a cheap organic substrate.
The transposition chain such as described with reference to FIG. 1, comprises a subharmonique mixer. As represented diagrammatically in FIG. 2, the subharmonique mixers customarily used in this type of system comprise two Schottky diodes 20 and 21 arranged in an antiparallel manner in one and the same package 23. Thus, in a known manner, if the two diodes 20, 21 are identical, the dc components of the currents I1 and I2 flowing around the diodes are zero and the component at the frequency 2FOL on the RF output is nonexistent. As represented in FIG. 2, the package 23 comprising the two diodes 20, 21 is connected to earth through a circuit 25 and to a local oscillator LO by way of a circuit 24 comprising a network of filters and an impedance matching circuit for matching to the frequency of the local oscillator LO. The outputs of the circuits 24 and 25 are connected at a common point P1 to the input of the package 23. In a symmetric manner, to the other input/output point of the package 23 are connected, through a circuit 26 comprising a network of filters and an impedance matching circuit, the RF signal, and through a circuit 27 comprising a network of filters and an impedance matching circuit, the IF signal. In practice, in mixers of the type of that of FIG. 2, the Schottky diodes arranged in antiparallel manner are never perfectly paired. Hence, this results in a residual component of dc current in the loop due to the imbalance which gives rise to a component at the frequency 2OL at the RF output.